ICU4C: Guard C++ public headers with C_SHOW_CPLUSPLUS_API
Description
Continue the discussion at ICU-20530, I provide a major cleanup without functionality change for icu public headers.
1. Remove all trailing spaces
2. Make the once macro consistent and unique and at format _U_HEADER_H_
3. Align file file prologue
4. Guard C++ using C_SHOW_CPLUSPLUS_API to allow include in C
